1. Field of the Invention
The present invention relates to a control device for guided travel of an unmanned vehicle, more specifically to a control device for guided travel of unmanned vehicle, in which the unmanned vehicle is guided to travel based on a signal sent from a GPS satellite.
2. Disclosure of the Related Art
In large-scale working sites, such as quarries to crush stone and mines, an unmanned dump truck traveling system is employed. In this system, unmanned dump trucks, namely, unmanned vehicles are guided to travel along a target travel course, and carry earth and sand.
In the working sites, it is significantly important for safety reasons to make the unmanned vehicle, which automatically runs, travel without interfering with other vehicles or obstacles.
When the unmanned vehicle is guided to travel, errors occur due to various factors. Thus, in addition to the target travel course, a guidable range is set as an area for safely guiding the vehicle to travel.
FIG. 1 is a diagram of a related art, showing a top view where an unmanned vehicle 10 is guided to travel.
The unmanned vehicle 10 measures a position Xi of the unmanned vehicle 10 itself, and controls such that a positional shift ΔX between a measured vehicle position Xi and each target point Xid, which is one of passing points sequentially set on a target travel course L, falls into an allowable range for controlling error Xc. Then, the guided travel is performed.
The vehicle position Xi of the unmanned vehicle 10 is measured with a GPS position measurement device provided to the unmanned vehicle 10. Specifically, the unmanned vehicle 10 receives a signal sent from a GPS satellite, and measures the position Xi of the vehicle 10 itself based on the received signal.
The accuracy of the positional measurement with the GPS position measurement device depends on arrangement or position of the GPS satellite or other factors, and varies dynamically. The positional information obtained from the signal sent from the GPS satellite includes position measurement accuracy information. The accuracy of the positional measurement with the GPS position measurement device can be known based on the position measurement accuracy information.
Japanese Patent Application Laid-open No. 2000-284830 discloses an invention in which, in addition to the GPS position measurement device, an unmanned vehicle is provided with a position measurement device using a dead reckoning, and the positional measurement is switched to the positional measurement with the dead reckoning at the time when the accuracy of the positional measurement with the GPS position measurement device becomes deteriorated.
Here, total guidance error X of the unmanned vehicle 10 is defined by the following expression.X (guidance error)=2Xp (position measurement error)+ΔX (positional shift)  (1),where the position measurement error 2Xp is a total value combining each of the right error Xp and the left error Xp set around the target travel course L.
On the other hand, a guidable range D in which the allowable range for control accuracy Xc is added to the position measurement error 2Xp is defined by the following expression.D (guidable range)=2Xp (position measurement error)+Xc (allowable range for controlling error)  (2)An area outside the guidable range is a safety zone, a zone for other vehicles, obstacles or an area where traveling is prohibited such as road shoulders.
For this reason, a condition for safely guiding the unmanned vehicle 10 to travel is:D (guidable range)>X (guidance error)  (3).On the other hand, if the guidance error X exceeds the guidable range D, in other words, if the positional shift ΔX exceeds the allowable range for controlling error Xc, the unmanned vehicle 10 is controlled to reduce the speed or be stopped for safety reasons because there is a possibility that the unmanned vehicle 10 goes into the safety zone, or goes beyond the safety zone and enters into the area where traveling is prohibited.
Conventionally, a size of the position measurement error 2Xp is equally set.
Here, a size of the guidable range D depends on topographical features of the working site. In a place where a margin for the traveling course of the unmanned vehicle 10 is tight, the guidable range D becomes narrow, and the allowable range for control accuracy Xc becomes narrow correspondingly (see the expression (2) above). Therefore, in a place where the width of the traveling course is tight, the positional shift ΔX of the unmanned vehicle 10 is likely to go beyond the allowable range of control accuracy Xc, which causes frequent speed reductions or stops of the unmanned vehicle 10, and a decrease in the productivity.
Additionally, in the conventional technique, the unmanned vehicle is equally stopped when information (status information or error information) on the accuracy of the positional measurement included in the positional measurement accuracy information sent from the GPS satellite indicates that “accuracy of the positional measurement is poor.” Therefore, the productivity becomes decreased in a situation where the accuracy of the positional measurement is poor.